PURPOSE: To solve the problem of bad contact of a connector, by installing a means which sends the output of a density detection oscillator, which oscillates when the bias voltage supplied to the developing electrode is supplied to the oscillator as the power source, to the outside of the developing apparatus under a noncontacted condition.

CONSTITUTION: One connector B of the two connectors connecting a developing device 1 and main body of a copying machine with each other is connected to a developing sleeve 4 and applies a bias voltage, and the other connector K is for earthing and used for producing a low-voltage power source for a toner density detecting oscillator 13. When toner to be detected, falls down into a cylindrical member 16, the inductance of a coil 14 installed around the cylindrical member 16 is changed and the oscillator detects the toner density. When the output of the oscillator 13 is introduced to a coil 20 at the transmission side, the oscillator 13 is magnetically coupled with a coil 25 at the reception side of the main body of the copying machine. Therefore, the density signal can be outputted outside without using any exclusive connector and the problem on bad contact of the connector is solved because the connection by means of the connectors B and K is relatively high voltage.